During normal electric arc welding the arc is formed between a pointed electrode (often made from tungsten) and the work-piece which servers as the second electrode often grounded (this form of discharge between two electrodes one of which may be grounded is called a two sided discharge). This requirement of a two sided electrode for welding is often the cause for problems associated with the electrical circuitry. If a one sided electrode is possible, many different possibilities otherwise not feasible in conventional arc welding become available to the welding engineer. For a comprehensive reference see Gas Tungsten Arc Welding Handbook by William H. Minnick, 1999 and Welders Handbook, R. Finch, Berkeley Publishing Group, NY, 1997.
Power Source
For two sided electrodes, although an alternating type (sine wave) power source is useful, arcs are normally not readily stabilized with sine wave AC. Arc re-ignition is difficult when there is a long electrode to work-piece distance. Special-purpose switched DC power sources are now becoming available to overcome such problems. By un-balancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability. Such electronics often add to the overall cost. In addition such processes have the disadvantage that arc starting is often a difficult process.
Electrode
The fact that two electrodes, one of which may be grounded adds to the cost of welding regardless of the fact whether they are consumable or not. Two electrodes are normally effectively required for welding and one of them is often the work piece. The arc or plasma-arc travels from one electrode to the other and if either electrode is removed the arcing (and thus the welding) stops. The electrode used commonly for the arc process for example is a costly e.g. tungsten-2% thoria an expensive material. Shielding gasses are also required. The normal combination of gases used are argon for the gas, with argon plus 2 to 5% hydrogen for the shielding gas, again all costly and somewhat dangerous because of the hydrogen.